During the combustion process, gasoline is oxidized and hydrogen (H) and carbon (C) combine with air. Various chemical compounds are formed including carbon dioxide (CO2), water (H2O), carbon monoxide (CO), nitrogen oxides (NOx), unburned hydrocarbons (HC), sulfur oxides (SOx), and other compounds.
Automobile exhaust systems include a catalytic converter that reduces emissions by chemically converting exhaust gas into carbon dioxide (CO2), nitrogen (N), and water (H2O). Exhaust gas oxygen (O2) sensors generate signals indicating the oxygen content of the exhaust gas. One O2 sensor monitors the oxygen level associated with the inlet of the catalytic converter.
The inlet O2 sensor provides a primary feedback signal to the fuel system. The signal that is generated by inlet O2 sensor is used to control the A/F ratio of the engine. Maintaining the A/F ratio at the chemically correct or stoichiometric A/F ratio improves the efficiency of the catalytic converter. A second or outlet O2 sensor monitors oxygen levels of the exhaust gas that exits the catalytic converter. The outlet O2 sensor provides a secondary feedback signal to the fuel system. An optimal control range of the outlet O2 sensor signal is defined by emission performance. The fuel system shifts an offset or bias of the inlet O2 sensor signal when the outlet O2 sensor signal is outside of a predetermined control range.
A/F ratio imbalance within individual cylinders of an engine and exhaust leaks can lead to undesired exhaust emission performance. As a result, it is necessary for a diagnostic system to identify A/F imbalance or leak conditions.